The present invention is concerned with systems for dispensing fluid in batches of a desired size, that is, a desired mass or volume.
The dispensing of fluids in batches of a desired size is a very common operation in many branches of industry. The term fluids in this connection covers a broad variety of flowable media such as true liquids, for example mineral oil products, beverages and liquid foodstuffs, gases, flowable solids such as fine powders, slurries etc. An example is the filling of containers for sale. The containers must be filled quickly to an exact amount. Repeated overfilling is as unacceptable for reasons of economy as repeated underfilling which might be considered fraudulous by customers or regulatory bodies. Incidents of container overflow because of excess overfilling, which can lead to contamination of production facilities, production stoppage and even dangerous situations such as fire hazard, must also be avoided.
A typical batch dispensing system comprises a fluid duct equipped with a flowmeter and an electrically controlled flow controller, such as a valve or a pump, for starting and stopping fluid flow to a dispensing nozzle or the like. The valve or pump is controlled by a control unit which receives the flowmeter""s output signal (the flow signal). The control unit includes a facility for setting a desired batch size, a start signal generator, an integrator or accumulator for integrating the flow signal over time and a comparator for generating a stop signal when the integrated or accumulated flow signal equals the desired batch size. The accumulator is reset to zero after each batch. Flowmeter, valve or pump and control unit may be separate or integrated units. The fluid to be dispensed is typically led to the dispensing system from a storage tank, a pressurized container, a feed pressure generating pump or the like via a length of tubing.
The starting and stopping of the fluid flow in a dispensing system of the kind described makes it difficult to dispense exact batches. Most flowmeters follow changes in flow rate only with a certain time lag. Thus the measured flow will be smaller than the actual flow at least for some time after start, and the mass or volume measured by integration of the measured flow will be less than the actual mass or volume dispensed. Valves and pumps also take a certain time to close or stop which means that the integrator must develop a stop signal ahead of the time when flow must actually stop to result in an exact batch.
These problems are addressed in U.S. Pat. No. 5,431,302. The document describes a batch dispensing system with a flow controlling valve wherein the actual overfill is averaged over a number of batches. The average overfill is then used to cause an earlier closure of the valve so that subsequent batches will be filled exactly.
However the batch metering problem is compounded by the fact that typically the feed pressure of the fluid to be dispensed and thus the maximum flow rate occurring in each individual batch is not constant over time. In the general examples given above, the feed pressure decreases with the fluid head in the storage tank, or with the pressure in the pressurized container, as they are increasingly emptied. In more complicated process environments quite erratic feed pressure variations may occur. This is a problem which cannot be solved with the approach of U.S. Pat. No. 5,431,302.
To address this problem, the present invention provides a system for dispensing batches of fluid which comprises flow controlling means for starting a flow of fluid in response to a start signal and for subsequently stopping the flow of fluid in response to a stop signal, a flowmeter for measuring a flow rate of the fluid and generating a flow signal corresponding thereto, and a batch controller for generating the start signal, the batch controller receiving the flow signal and generating the stop signal in response thereto. The batch controller comprises storage means for storing a target batch size representation, integrating means for integrating the flow signal over time to develop a measured batch size representation, comparator means for comparing the target batch size representation with measured batch size representation and generating the stop signal when they are equal, and modifying means for modifying at least one of the target batch size representation and the measured batch size representation in response to the flow signal.
The system according to the invention alleviates the problem of variations in feed pressure by providing a correction in response to the actually observed flow, instead of a correction based on a forecast from past history.
The correction principles associated with the invention may also applied to batch controllers for controlling batch dispensing systems, and to flowmeters for use in such systems.